1. Field of the Invention
The present invention relates to a transmitter and a distortion compensation method to be used therefor. More particularly, the invention relates to a distortion compensating method in a transmitter having a pre-distortion type linearizer.
2. Description of the Related Art
Conventionally, the transmitter of this type is constructed with a transmission signal generating portion 21, a pre-distortion type linearizer 22, a transmitter 23, a directional coupler 24, an antenna 25, a power calculator 26, a compensation value calculating means 27 and a demodulation means 28, as shown in FIG. 7.
Here, since a transmission signal is distorted due to non-linear characteristics or an amplifier or the like, in case of the transmitter 23 alone, the pre-distortion type linearizer 22 is provided between an output of the signal generating portion 21 and an input of the transmitter 23.
The pre-distortion type linearizer 22 performs correction for mutually canceling distortion component and correction data component generated in the transmitter 23. By this, distortion of an output waveform of the transmitter 23 can be improved. The directional coupler 24 divides an RF signal. Host of the power thereof becomes an output of the antenna 25. However, a part of the power is input to the demodulating means 28. The power calculator 26 calculates an instantaneous power of a base band signal.
As a generation method of the compensation data input to the pre-distortion type linearizer 22, there is a method to return a part of the output of the transmitter 23 to the base band signal by the demodulator 28 and to arithmetically derive the distortion component from this signal and the result of the power calculator 26 by the compensation value calculating means.
In the distortion compensation method in the conventional transmitter set forth above, since the distortion component is arithmetically derived by returning a part of the transmitter to the base band signal by the demodulation means and calculating the distortion component from this signal and the result of the power calculator by the compensation value calculating means, a scale of the circuit becomes large to also increase current consumption.